1. Field of Endeavor
The present invention relates to a press plate for tightening the metal sheets of a stator core of an electric machine. In particular the present invention relates to the stator core of a rotating electric machine such as an electric generator.
2. Brief Description of the Related Art
With reference to FIG. 1, rotating electric machines such as electric generators have a stator core 1 with an annular shape; within the stator core 1, a rotor (not shown) is housed.
The stator core 1 is made of a plurality of metal sheets 2 grouped in packets 3; each packet 3 is separated from the adjacent packets via spacers (not shown), to define stator cooling channels 4.
In order to keep all the metal sheet packets 3 together, press plates 5 are provided at the two opposite stator core ends, to fix and press the metal sheet packets 3 together.
In order to connect the press plates 5 to the metal sheet packets 3, key bars 6 (that are under tension load) are welded to the metal sheets 2; in addition, straps 7 are welded at the ends of the key bars 6 and are inserted in recessed seats 8 of the press plates 5.
Traditionally, in order to guarantee the stator core tightening, the metal sheet packets 3 are pre-compressed (pre-compression is kept over the lifetime of the generator); generally pre-compression is achieved by vibration and pressing.
In addition, in order to obtain an additional compressive force at the inner diameter of the core, the press plates 1 are provided with a tapered wall 10 that is pressed against the end metal sheet packets, such that the tapered wall 10 deforms to have all its portions in contact with the end metal sheet packets.
This configuration allows a large pre-compression force to be applied on the metal sheet packets 3, such that they are well tightened in all conditions; nevertheless, it also causes a very high stress to be induced within the press plates 5.
In addition, as shown in FIG. 1, a press plate wall 11 opposite the tapered wall 10 has threaded cylindrical holes 13, arranged to receive screws connecting the end winding supports 15.
These holes 13 cause high stress peaks within the press plates 5 in the zones close to them; in particular, tests showed that these stress peaks may also be larger than three times the stress of press plate zones without holes 13.
It is clear that these high stress peaks are very troubling and should be reduced.